The goal of this proposal is to determine the structural basis for the complex functional properties of G protein coupled receptors (GPCRs) using the 22 adrenergic receptor (22AR) as a model system. GPCRs are the largest family of receptors for hormones and neurotransmitters and therefore the largest group of targets for new therapeutics for a very broad spectrum of diseases including neuropsychiatric, cardiovascular, pulmonary and metabolic disorders, cancer and AIDS. GPCRs exhibit complex and diverse signaling behaviors. A single GPCR can activate more than one G protein subtype as well as G protein independent signaling pathways such as arrestins. Many GPCRs exhibit basal, agonist independent activity. When considering one of the several possible downstream signaling pathways, a drug acting at the orthosteric binding pocket may exhibit one of four different efficacy profiles. It may behave as an inverse agonist, suppressing basal activity, a full agonist, maximally activating the pathway, a partial agonist, promoting submaximal activity even at saturating concentrations, or a neutral antagonist, having no effect on basal signaling, but blocking the binding of other orthosteric ligands. The efficacy profile of a ligand may differ for different signaling pathways. Finally, the functional response to an orthosteric ligand can be modulated allosterically by other (allosteric) ligands, protons, ions, lipids, peptides and proteins, including other GPCRs. This remarkable functional versatility is due in part to the dynamic character of GPCRs. Understanding how GPCRs work requires both high-resolution structures and approaches to characterize protein dynamics such as Nuclear Magnetic Resonance Spectroscopy (NMR) and Electron Paramagnetic Resonance Spectroscopy (EPR). Specific Aims include: Aim 1. Develop methods and experimental tools for applying NMR and EPR spectroscopy to characterize the conformational dynamics of the 22AR. Aim 2. Examine effect of ligands from different efficacy and affinity classes on 22AR structure and dynamics. Aim 3. Examine the effect of oligomerization on 22AR structure and dynamics. Aim 4. Determine the high-resolution structure of the 22AR-arrestin complex. PUBLIC HEALTH RELEVANCE: The goal of this proposal is to determine the mechanism by which G protein coupled receptors (GPCRs) respond to hormones and neurotransmitters, and modify the function of cells. This information will facilitate the process of drug discovery for GPCRs, which are the largest family of membrane proteins in the human genome. Drugs acting on GPCRs can have an impact on a broad spectrum of diseases including: cardiovascular disease, pulmonary disease, inflammation, diabetes and obesity, behavioral disorders and Alzheimer's disease.